feat: add upload policies, daily limits, and storage quotas

- Add environment variables to configure anonymous uploads, daily upload caps, and default user storage limits.
- Update config loader to parse and validate the new settings.
- Implement backend logic to track daily usage and active storage per user.
- Update README and `.env.example` to document the new settings and admin panels.

backend/libs/config/config.go

@@ -28,6 +28,16 @@ type Config struct {
 	ThumbnailEnabled bool
 	ThumbnailEvery   time.Duration
 	MaxUploadSize    int64
+	DefaultSettings  SettingsDefaults
+}
+
+type SettingsDefaults struct {
+	AnonymousUploadsEnabled bool
+	AnonymousMaxUploadMB    float64
+	AnonymousDailyUploadMB  float64
+	UserDailyUploadMB       float64
+	DefaultUserStorageMB    float64
+	UsageRetentionDays      int
 }
 
 func Load() (Config, error) {
@@ -49,6 +59,14 @@ func Load() (Config, error) {
 		ThumbnailEnabled: envBool("WARPBOX_THUMBNAIL_ENABLED", true),
 		ThumbnailEvery:   envDuration("WARPBOX_THUMBNAIL_EVERY", time.Minute),
 		MaxUploadSize:    envMegabytes("WARPBOX_MAX_UPLOAD_SIZE_MB", 2048), // 2 GiB default.
+		DefaultSettings: SettingsDefaults{
+			AnonymousUploadsEnabled: envBool("WARPBOX_ANONYMOUS_UPLOADS_ENABLED", true),
+			AnonymousMaxUploadMB:    envMegabytesFloat("WARPBOX_ANONYMOUS_MAX_UPLOAD_MB", 512),
+			AnonymousDailyUploadMB:  envMegabytesFloat("WARPBOX_ANONYMOUS_DAILY_UPLOAD_MB", 2048),
+			UserDailyUploadMB:       envMegabytesFloat("WARPBOX_USER_DAILY_UPLOAD_MB", 8192),
+			DefaultUserStorageMB:    envMegabytesFloat("WARPBOX_DEFAULT_USER_STORAGE_MB", 51200),
+			UsageRetentionDays:      envInt("WARPBOX_USAGE_RETENTION_DAYS", 30),
+		},
 	}
 
 	if cfg.BaseURL == "" {
@@ -57,6 +75,13 @@ func Load() (Config, error) {
 	if cfg.MaxUploadSize <= 0 {
 		return Config{}, fmt.Errorf("WARPBOX_MAX_UPLOAD_SIZE_MB must be positive")
 	}
+	if cfg.DefaultSettings.AnonymousMaxUploadMB <= 0 ||
+		cfg.DefaultSettings.AnonymousDailyUploadMB <= 0 ||
+		cfg.DefaultSettings.UserDailyUploadMB <= 0 ||
+		cfg.DefaultSettings.DefaultUserStorageMB <= 0 ||
+		cfg.DefaultSettings.UsageRetentionDays <= 0 {
+		return Config{}, fmt.Errorf("upload policy settings must be positive")
+	}
 
 	return cfg, nil
 }
@@ -109,6 +134,19 @@ func envBool(key string, fallback bool) bool {
 	return parsed
 }
 
+func envInt(key string, fallback int) int {
+	value := strings.TrimSpace(os.Getenv(key))
+	if value == "" {
+		return fallback
+	}
+
+	parsed, err := strconv.Atoi(value)
+	if err != nil {
+		return fallback
+	}
+	return parsed
+}
+
 func envMegabytes(key string, fallback float64) int64 {
 	value := strings.TrimSpace(os.Getenv(key))
 	if value == "" {
@@ -122,7 +160,27 @@ func envMegabytes(key string, fallback float64) int64 {
 	return parsed
 }
 
+func envMegabytesFloat(key string, fallback float64) float64 {
+	value := strings.TrimSpace(os.Getenv(key))
+	if value == "" {
+		return fallback
+	}
+	parsed, err := parseMegabytesFloat(value)
+	if err != nil {
+		return fallback
+	}
+	return parsed
+}
+
 func parseMegabytes(value string) (int64, error) {
+	sizeMB, err := parseMegabytesFloat(value)
+	if err != nil {
+		return 0, err
+	}
+	return megabytesToBytes(sizeMB), nil
+}
+
+func parseMegabytesFloat(value string) (float64, error) {
 	normalized := strings.TrimSpace(value)
 	normalized = strings.TrimSuffix(normalized, "MB")
 	normalized = strings.TrimSuffix(normalized, "Mb")
@@ -137,7 +195,7 @@ func parseMegabytes(value string) (int64, error) {
 		return 0, fmt.Errorf("megabyte value must be positive")
 	}
 
-	return megabytesToBytes(sizeMB), nil
+	return sizeMB, nil
 }
 
 func megabytesToBytes(sizeMB float64) int64 {